This invention relates generally to transfer presses and, more particularly, to transfer presses of the type wherein workpieces are stamped and/or drawn in a progressive manner at successive working stations, the shape, size and/or configuration of the workpiece changing from station to station.
Transfer presses of the type wherein a workpiece is progressively drawn at successive drawing stations are of course well known. More particularly, it is common in such transfer presses for the outer shape of the workpiece to change from station to station. In such cases, movable fingers have been utilized to assist in transferring the workpiece from station to station.
Generally, in transfer presses where the shape of the workpiece changes from station to station, the drawing dies for the particular job are arranged in a rectilinear fashion, i.e., the dies are arranged in the die block with a common straight centerline. A plurality of pairs of transfer fingers are provided, the function of each pair of transfer fingers being to grasp a workpiece subsequent to its being cut or formed at a working station, transfer the workpiece to the next working station, release the same in conjunction with a succeeding drawing operation and return to the first working station in a manner timed so that the next workpiece has been already formed whereupon the pair of transfer fingers can grasp this workpiece and continue through the next transfer cycle. The transfer fingers of a particular transfer finger pair are located on opposite sides of the common centerline of the die block and means are provided for moving the transfer fingers in timed coordination with the movement of the draw punches, firstly towards each other at a first work station whereupon the workpiece formed therein is grasped, and secondly in a rectilinear direction transverse to this first inward motion to transfer the workpiece to the next die. Upon the grasped workpiece finally arriving at the next working station, the transfer fingers now move away from each other to release the workpiece for subsequent working operations at this next die. Finally, the transfer fingers return to the first working station by again moving in a rectilinear fashion back to the first working station in preparation for transferring the subsequently formed workpiece in a similar manner.
Transfer presses of the type described above are well known and are currently manufactured by companies such, for example, at the U.S. Baird Corporation of Stratford, Connecticut and M. S. Willet, Inc. of Cockeysville, Maryland. Such transfer presses can be termed "linear transfer presses" to denote the fact that the dies are arranged in the holding block in a linear fashion as described above. The workpiece is moved from station to station in a straight line and the transfer fingers similarly move only linearly during operation of the press. Prior to the present invention, transfer presses of the "linear" type have been exclusively used in connection with jobs wherein the outer shape of the workpiece changes from station to station.
However, linear transfer presses have certain disadvantages. For example, in jobs which require a large number of drawing operations and, correspondingly, a large number of drawing dies, the rectilinear arrangement of the dies in the die block requires a large amount of space on the floor to be allocated to the press. Not only does this take up valuable space but, further, the full tonnage capabilities of the press cannot be most efficiently utilized over a long die block. Linear transfer presses are relatively slow due to the fact that the workpiece transfer operation is most often the limiting factor in the rate of production. The linear arrangement of the dies also generally necessitates a relatively long pitch, i.e. a substantial distance between adjacent dies. However, the longer the distance which the workpiece must travel between dies, in addition to resulting in a longer press, also decreases the speed of the press, i.e., fewer strokes per minute can be achieved.
Further, in conventional linear transfer presses, the pitch and center spacing of successive drawing stations is fixed once the press is designed. In other words, the working stations are integral parts of the press structure. This is disadvantageous in that the flexibility of the linear transfer press is severely limited, i.e., a linear transfer press can be used only for transfer press operations and cannot be converted not use as a conventional press. Similarly, it is generally for possible for a more complex progressive die to be used in conventional transfer presses due to the lack of availability of space therefor.
Linear transfer presses which are currently available are fairly complex in construction and are quite expensive.
Certain conventional linear transfer presses have other important disadvantages. More particularly, certain linear transfer presses do not have the capability of positively controlling the workpiece, i.e., positively grasping the workpiece, in a continuous manner from the initial blanking station to the final working station. In some cases, the workpiece will "float" either during the transfer operation or during part of the drawing operation thereby preventing close tolerances from being obtained and often resulting in misfeeds and the like. In many cases, the tooling, e.g., the dies, are not easily accessible for purposes of replacement thereby requiring a large amount of time for tool replacement either during normal maintenance or during job changes. A further disadvantage exists in some linear transfer presses in that the transfer apparatus, e.g., the plurality of pairs of transfer fingers, often at some time during operation of the machine pass through the path of the tooling. In these cases, it is not uncommon for the tooling and transfer apparatus to collide with each other requiring expensive repair and down-time for the machine.
The workpiece transfer apparatus used in certain conventional linear transfer presses are not entirely satisfactory. Thus, the transfer fingers are often only spring loaded and, consequently, must rub across the workpiece to open. This leads to an inaccurate positioning of the workpiece and, therefore, improper tolerances. The proper operation of the transfer apparatus is, of course, critical to accurate timing and overall operation of the press.